This invention is directed to glass compositions for ophthalmic applications which, in addition to exhibiting low absorption of radiation in the visible region of the spectrum (i.e., the essential absence of coloration), demonstrate an improved color stability when lenses prepared therefrom have anti-reflective coatings applied thereupon.
When anti-reflective coatings are applied to certain commercially available glass using glow discharge equipment, an objectionable brown coloration tends to develop in the glass. During the process of producing an anti-reflective layer, glass, in the form of a corrective lens, is generally subjected to a discharge of a radiation of a short wavelength which induces an undesirable brown coloring. As a result, many commercially available glasses suffer from color instability when an anti-reflective layer is applied.
There are many examples of commercially available ophthalmic glass compositions. For example, U.S. Pat. No. 2,068,801 (Armistead) discloses the production of borosilicate glasses containing not less than 5% B.sub.2 O.sub.3, not more than 10% alkali metal oxide, not less than 2% CeO.sub.2, and not less than 7% BaO and/or ZnO, each component being present in weight percent. Al.sub.2 O.sub.3 in an amount of 5% is cited in one of the working examples.
U.S. Pat. No. 2,657,146 (Kreidl) claims borosilicate glasses resistant to coloration caused by exposure to high energy sources consisting, in weight percent, of 60-70 SiO.sub.2, 10-15 Na.sub.2 O, 0-2 SrO, 10-20 B.sub.2 O.sub.3, 0.2-2 CeO.sub.2, 0-2 BaO, 1-5 ZnO, and 0-2 Al.sub.2 O.sub.3.
U.S. Pat. No. 3,951,671 (Parry et al.) describes borosilicate glasses for ophthalmic applications comprising, in weight percent, of 68-75 SiO.sub.2, 16-22 Na.sub.2 O, 0.05-0.5 ZnO, 4.5-8 Al.sub.2 O.sub.3, 0.5-2.8 CeO.sub.2, 0.05-0.8 SrO, and 0-4 B.sub.2 O.sub.3.
U.S. Pat. No. 4,565,791 (Boudot et al.) is directed to ophthalmic glasses exhibiting a cutoff of ultraviolet radiation at 2 mm thickness between 310-335 nm consisting essentially, in weight percent, of 49-71 SiO.sub.2, 5-26 B.sub.2 O.sub.3, 1.8-6 TiO.sub.2, 0-14 Al.sub.2 O.sub.3, 0-5.5 ZrO.sub.2, 0-4 Li.sub.2 O, 0-16 Na.sub.2 O, 0-20 K.sub.2 O, 8-20 Li .sub.2 O+Na.sub.2 O+k.sub.2 O, 0-0.7 As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3, 0-1 Cl and/or Br, 0-4 and/or MgO and/or BaO and/or ZnO.
U.S. Pat. No. 5,017,521 (Yale et al.) is drawn to glasses designed for covers for solar cells, the glass compositions comprising, in weight percent, of 60-78 SiO.sub.2, 0.5-3 Al.sub.2 O.sub.3, 10-25 B.sub.2 O.sub.3, 0-2 ZrO.sub.2, 3.5-6 Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O, .ltoreq.3 Al.sub.2 O.sub.3 +ZrO, 2-6.5 CeO.sub.2, 0-2 TiO.sub.2, 0.25-8 As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3, 0-5 MgO and/or CeO and/or SrO and/or BaO, and 0-5 ZnO.
U.S. Pat. No. 5,219,801 (Shorrock et al.) presents glasses utilized as protective covers for solar cells, the glass compositions consisting essentially, in mole percent, of 57-77 SiO.sub.2, 7.6-10.6 Li.sub.2 O+Na.sub.2 O+K.sub.2 O, 0-8.5 Al.sub.2 O.sub.3, 0.7-2 CeO.sub.2, 4.6-23 B.sub.2 O.sub.3, 0-0.3 Sb.sub.2 O.sub.3,81-91 SiO.sub.2 +Al.sub.2 O.sub.3 +B.sub.2 O.sub.3, 0-0.03 As.sub.2 O.sub.3, 0-1.5 Li.sub.2 O, 0-2 CeO.sub.2, 2-7.5 Na.sub.2 O, 0-2 F.sub.2, 0-7 MgO and/or CaO and/or SrO and/or BaO and/or PbO.
There is also available, a commercially available ophthalmic glass which exhibits the following properties:
(a) a refractive index of 1.523; PA1 (b) an Abbe number between 55-62; PA1 (c) a density of 2.60 grams/cm.sup.3 ; PA1 (d) a linear coefficient of thermal expansion over the temperature range of 0.degree.-300.degree. C. of 93.times.10.sup.-7 /.degree.C.; PA1 (e) a softening point of 710.degree. C.; PA1 (f) a transmission of radiation at a wavelength of 400 nm through a glass thickness of 2 mm of 90.5%; and PA1 (g) a cutoff of ultraviolet radiation, defined as the wavelength at which the transmission thereof is equal to 1% for a thickness of 2 mm, at a wavelength of 328 nm. PA1 (1) a refractive index of 1.523.+-.0.005; PA1 (2) an Abbe number between 55-62; PA1 (3) a density less than 2.7 grams/cm.sup.3 ; PA1 (4) a linear coefficient of thermal expansion over the temperature range of 0.degree.-300.degree. C. between 88-95.times.10.sup.-7 /.degree.C.; PA1 (5) a softening point between 680.degree.-730.degree. C.; PA1 (6) a transmission of radiation at a wavelength of 400 nm through a glass thickness of 2 mm greater than 85%; PA1 (7) a cutoff of ultraviolet radiation, i.e., a transmission equal to 1% at a thickness of 2 mm, between 310-335 nm; PA1 (8) resistance to the development of coloration upon being exposed to the glow discharge occurring in common anti-reflective coating equipment; and, most preferably, PA1 (9) resistance to the development of solarization.
The above glass shows a sensitivity to the treatment used to apply an anti-reflective coating in a glow-discharge equipment; namely, the occurrence of an undesirable brown coloration.
Because such glass may be used as the major portion of a multifocal ophthalmic lens which will be fused to particular existing commercial segments, it is critical that the ophthalmic glass maintains its coefficient of thermal expansion, as well as its softening point within their respective ranges.